mention pr folder in whatsnew and CONTRIBUTING

CONTRIBUTING.md

@@ -28,6 +28,10 b' Some guidelines on contributing to IPython:'
 * Pull Requests should be tested, if feasible:
     - bugfixes should include regression tests
     - new behavior should at least get minimal exercise
+* New features and backwards-incompatible changes should be documented by adding
+  a new file to the [development](docs/source/whatsnew/development) directory,
+  see [the README.md there](docs/source/whatsnew/development/README.md) for
+  details.
 
 [Travis](http://travis-ci.org/#!/ipython/ipython) does a pretty good job testing IPython and Pull Requests,
 but it may make sense to manually perform tests (possibly with our `test_pr` script),

docs/source/whatsnew/development.rst

@@ -4,6 +4,13 b''
 
 This document describes in-flight development work.
 
+.. warning::
+
+    Please do not edit this file by hand (doing so will likely cause merge
+    conflicts for other Pull Requests). Instead, create a new file in the
+    `docs/source/whatsnew/pr` folder
+
+
 - `%%capture` cell magic now captures the rich display output, not just
   stdout/stderr